The present invention relates to wide-band radio frequency (RF) receivers, and more particularly to a system for calibrating wide-band RF receivers for amplitude flatness and phase linearity.
Increased data bandwidth requirements have driven a need for RF receivers that not only receive signals with very wide-band modulation, but also that maintain a flat amplitude and linear phase response over a wide bandwidth. As shown in FIG. 1, a wide-band digital RF receiver 10 has an input for receiving multiple RF signals from an antenna or other source. An image reject filter 12 band-limits the signal prior to a mixer 14. The mixer 14 down-converts the band-limited signal, using a local oscillator 16, to an intermediate frequency (IF). The IF signal from the mixer 14 is input to an amplifier 18 and filter 20, and then digitized in response to a receiver clock by an analog-to-digital converter (ADC) 22 to produce a stream of digitized samples. The samples are then mathematically measured or demodulated by algorithms in a digital signal processor (DSP) 24 to produce results for the input signals.
The RF receiver 10 is usually calibrated as part of the manufacturing process, and a finite impulse response (FIR) filter 26 is placed at the output of the ADC 22 prior to input of the digital sample stream to the DSP 24. The FIR filter coefficients are set by the manufacturer's calibration process, and serve to correct for amplitude flatness and phase linearity under those ideal conditions. However, each of the mixers, filters, amplifiers and other circuitry in the signal path from the input to the DSP 24 contribute amplitude ripples and group delay ripples, i.e., deviations from linear phase. These active components often exhibit temperature dependence in gain as well as changes in their frequency response. Further, as components age the amplitude and phase response also may be affected. It is often required to improve amplitude accuracy, amplitude flatness and phase linearity of the RF receiver 10.
What is desired is a method of calibrating the RF receiver at the time of use, i.e., at run-time, to correct amplitude flatness and phase linearity for temperature and other environmental dependencies in order to improve the accuracy of the measurement or demodulation algorithms used in the DSP.